If for example your LAN is configured to have IP adresses in the range x.x.x.1 to x.x.x.255, you will put x.x.x.0 in the network line.

"address" is the IP you want the RPi will assume (in the proper range, as described above). pay attention not to use an IP already used by another device in your LAN or that can be assigned to a device by your router by DHCP (set the DHCP range of the router wisely in order to avoid potential overlaps).

"netmask" will "always" be 255.255.255.0

gateway is usually x.x.x.1 (your router IP or the one given by your ISP)

You now need to restart the network:

pi@raspberry:sudo /etc/init.d/networking restart

You may now be disconnected from your RPi if you are working through the network. Connect again to the RPi using the static IP you chose and you should now be fine.